The present invention provides a digital tachometer for an internal combustion engine having an ignition coil with an input primary low voltage lead and an output secondary high voltage lead.
The invention arose from efforts to develop a reliable and highly accurate tachometer for marine drive engines. It was further desired to provide a universal tachometer usable for both a two cycle engine, such as an outboard drive, and for a four cycle engine, such as a stern drive.
The tachometer of the invention senses pulses on the primary of the ignition coil, to avoid the high voltage and inference present on the secondary lead of the coil. Signal conditioning circuitry is provided to further isolate and condition both four cycle primary pulses and two cycle primary pulses. Further signal conditoning circuitry is provided for ignoring transients and the like, particularly with breaker point type ignitions. A precision timer provides an accurately controlled delay for ignoring false or transient pulses and the like. A phase-locked loop responds to the pulses and outputs a pulse train having a frequency as a function thereof. A counter and display circuit counts the pulses in the pulse train as a function of time and displays same to indicate revolutions of the engine per unit time.
A battery saver circuit is provided for supplying battery power to various components only when the primary pulses provide a voltage above a give threshold corresponding to a given engine speed. This extends battery life. A simple single switch controls both connection/disconnection of the battery in the battery saver circuit and connection of the timing circuitry to either of the two or four cycle conditioning circuitry or to an OFF position. This facilitates ease of user operation.